1. Field of the Invention
This invention relates to a deflection-distortion corrector for cathode-ray tube (CRT) display unit, and more specifically to such a corrector using permanent magnets.
2. Description of the Prior Art
For the deflection of electron beam in CRT display units, a system known as electromagnetic deflection is predominantly used. The system, as typically shown in FIG. 2, employs horizontal and vertical deflection coils 3 arranged around the neck of the CRT 2 having a phosphor screen 1 and supplied with a current to deflect the path of electron beam 4 scanning the screen. Close to the deflection coils, there is usually provided a deflection yoke of a high-permeability ferrite core. It is common with such deflection system that the distance between the center of deflection and the center of the image is not always equal to the radius of curvature of the image and that, in designing the display device, the deflecting magnetic field distribution is chosen as a compromise between the resolution and the degree of pincushion or barrel distortion involved. In many cases, for these and other reasons, mere combination of a CRT and deflection yoke-coil assembly would cause the scanning to produce a pincushion distortion as indicated at (e) in FIG. 3 or a barrel distortion as at (f) in place of the normal rectangle at (a) of the same figure.
A countermeasure usually taken is a system of electronic circuits for adjusting the deflecting current to the optimum value for each of the horizontal and vertical coordinate points, a system of permanent magnets arranged close to the deflection coils to correct the residual distortion in each magnetic field they generate in the space, or both. However, if the residual distortion were to be corrected with high accuracy, the electronic circuit system would become too expensive. The permanent magnet system, on the other hand, is less expensive because they use fixed magnets, but permanent magnets of proper length and strength in proper arrangement are so complex to design that high-accuracy deflection distortion correction by the system is seldom realized. The permanent magnet system in use will now be explained by reference to FIGS. 1 and 4 illustrating the present invention. Around the tubular wall near the neck of a CRT 10, there are mounted a horizontal deflection coil (13), a vertical deflection coil 12, and a conical deflection yoke 11. To correct deflection distortions, four bar-shaped permanent magnets 14 are also disposed (FIG. 4). The visual presentation on a CRT display unit is, ideally, required to be of good linearity as represented by the raster at (a) of FIG. 3. In reality, correction by the permanent magnet system leaves residual distortions as at (b), (c), or (d) in FIG. 3, necessitating adjustments in the directions of the arrows. An attempt to combine the system with another corrector means of electronic circuits presents other problems of inevitable residual distortion where weight is placed on cost reduction, or very high cost of complete correction. Precise correction by the electronic circuits alone is again extremely costly.
Another system has recently been proposed which uses, in addition to the permanent magnets 14, much smaller permanent magnets surrounding the deflection coils to effect fine adjustments for correction of distortion with high accuracy. The system produces magnetic fields for fine adjustments which are variable in magnitude but are fixed in direction. The fields cannot be arbitrarily given any desired direction or magnitude, with the consequence that the path of correction the spot on the screen follows is circular or elliptical and correction cannot be achieved as intended. The present invention is concerned with an improvement in the correction technique of this character.